Use of doped silicon dioxide in the fabrication of solar cells

ABSTRACT

In one embodiment, a method of forming doped regions in a substrate of a back side contact solar cell includes the steps of depositing a first doped oxide layer on a back side of a substrate, depositing a first undoped oxide layer over the first doped oxide layer, diffusing a first dopant from the first doped oxide layer into the substrate to form a first doped region in the substrate, and diffusing a second dopant into the substrate by way of a front side of the substrate, wherein the diffusion of the first dopant and the second dopant into the substrate are performed in-situ. The method may further include the steps of patterning the first doped and undoped oxide layers to expose portions of the back side of the substrate and depositing a second doped and undoped oxide layers on the back side of the substrate.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.10/946,564, filed on Sep. 21, 2004, now U.S. Pat. No. 6,998,288 whichclaims the benefit of U.S. Provisional Application No. 60/508,772,entitled “Use Of Doped Silicon Dioxide In The Fabrication Of SolarCells,” filed by David D. Smith, Michael J. Cudzinovic, and KeithMcIntosh on Oct. 3, 2003. The aforementioned disclosures areincorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to solar cells, and moreparticularly but not exclusively to methods and apparatus forfabricating solar cells.

2. Description of the Background Art

Solar cells are well known devices for converting solar radiation toelectrical energy. They may be fabricated on a semiconductor wafer usingsemiconductor processing technology. Generally speaking, a solar cellmay be fabricated by forming p-doped and n-doped regions in a siliconsubstrate. Solar radiation impinging on the solar cell creates electronsand holes that migrate to the p-doped and n-doped regions, therebycreating voltage differentials between the doped regions. In a back sidecontact solar cell, the doped regions are coupled to metal contacts onthe back side of the solar cell to allow an external electrical circuitto be coupled to and be powered by the solar cell. Back side contactsolar cells are also disclosed in U.S. Pat. Nos. 5,053,083 and4,927,770, which are both incorporated herein by reference in theirentirety.

Just like in most manufacturing processes, each step employed in themanufacture of solar cells has an associated cost. Thus, techniques forsimplifying the fabrication of solar cells are generally desirable.

SUMMARY

In one embodiment, a method of forming doped regions in a substrate of aback side contact solar cell includes the steps of depositing a firstdoped oxide layer on a back side of a substrate, depositing a firstundoped oxide layer over the first doped oxide layer, diffusing a firstdopant from the first doped oxide layer into the substrate to form afirst doped region in the substrate, and diffusing a second dopant intothe substrate by way of a front side of the substrate, wherein thediffusion of the first dopant and the second dopant into the substrateare performed in-situ. The method may further include the steps ofpatterning the first doped and undoped oxide layers to expose portionsof the back side of the substrate and depositing a second doped andundoped oxide layers on the back side of the substrate.

These and other features of the present invention will be readilyapparent to persons of ordinary skill in the art upon reading theentirety of this disclosure, which includes the accompanying drawingsand claims.

DESCRIPTION OF THE DRAWINGS

FIGS. 1–7 schematically illustrate a method of fabricating a solar cellin accordance with an embodiment of the present invention.

The use of the same reference label in different drawings indicates thesame or like components. Drawings are not necessarily to scale unlessotherwise noted.

DETAILED DESCRIPTION

In the present disclosure, numerous specific details are provided suchas examples of apparatus, process parameters, materials, process steps,and structures to provide a thorough understanding of embodiments of theinvention. Persons of ordinary skill in the art will recognize, however,that the invention can be practiced without one or more of the specificdetails. In other instances, well-known details are not shown ordescribed to avoid obscuring aspects of the invention.

The present disclosure relates to the fabrication of solar cells. Solarcell fabrication processes are also disclosed in the followingcommonly-assigned disclosures, which are incorporated herein byreference in their entirety: U.S. application Ser. No. 10/412,638,entitled “Improved Solar Cell and Method of Manufacture,” filed on Apr.10, 2003 by William P. Mulligan, Michael J. Cudzinovic, Thomas Pass,David Smith, Neil Kaminar, Keith McIntosh, and Richard M. Swanson; andU.S. application Ser. No. 10/412,711, entitled “Metal Contact StructureFor Solar Cell And Method Of Manufacture,” filed on Apr. 10, 2003 byWilliam P. Mulligan, Michael J. Cudzinovic, Thomas Pass, David Smith,and Richard M. Swanson.

FIGS. 1–7 schematically illustrate a method of fabricating a solar cellin accordance with an embodiment of the present invention.

In FIG. 1, an N-type FZ silicon wafer 102 is processed to a thickness ofabout 280 μm. Wafer 102 serves as the substrate of the solar cell beingfabricated. Wafer 102 has a front side 101 and a back side 103. Frontside 101 is the side of the solar cell configured to receive solarradiation, and thus faces the sun in operation. Wafer 102 is thinned toa thickness of about 240 μm using a process that also etches damagesfrom the surfaces of the wafer. The aforementioned process is alsoreferred to as “bath damage etch/critical clean,” and, in oneembodiment, comprises a wet etch using potassium hydroxide (e.g., twoparts potassium hydroxide to water).

In FIG. 2, a boron-doped silicon dioxide (SiO₂) (“BSG”) 104 is depositedon the back side of wafer 102. In one embodiment, boron-doped silicondioxide 104 is deposited to a thickness of about 1000 Angstroms using anatmospheric pressure chemical vapor deposition process (APCVD). Thethickness of boron-doped silicon dioxide 104, and other materialsdisclosed herein, may be varied depending on the application. Forexample, boron-doped silicon dioxide 104 may also be deposited to athickness of about 500 Angstroms by APCVD. Thereafter, an undopedsilicon dioxide 106 is deposited over boron-doped silicon dioxide 104.In one embodiment, undoped silicon dioxide 106 is deposited to athickness of about 1200 Angstroms also by APCVD. The use of a depositionprocess, such as APCVD, to deposit the boron-doped silicon dioxide andthe undoped silicon dioxide advantageously allows for one-sideddeposition. This is particularly useful in the manufacture of back sidecontact solar cells where only one side of the solar cell is textured.Note that depending on the application, doped silicon dioxide 104 mayalso be doped with P-type dopants other than boron.

In FIG. 3, a mask 108 is formed over the silicon dioxide stackcomprising the boron-doped and undoped silicon dioxide. Mask 108 will beused in a subsequent etch process (see FIG. 4) exposing the would be N+region of the wafer. Mask 108 may comprise an ink formed by a printingprocess such as screen printing, pad printing, or ink-jet printing. Inone embodiment, the ink comprises a particle-free ink, and may be of thesame type as the Coates ER-3070 ink available from Coates Screen of St.Charles, Ill. The particle-free ink may be applied by screen printing.The use of a printing process, such as screen printing, is advantageousin that sufficiently small feature sizes can be achieved with lowerprocess costs.

In FIG. 4, mask 108 is employed in patterning the silicon dioxide stackcomprising boron-doped silicon dioxide 104 and undoped silicon dioxide106. In one embodiment, the silicon dioxide stack is patterned using awet etch process comprising buffered hydrofluoric acid. The wet etchprocess uses the silicon of wafer 102 as an etch stop, and etchesportions of the silicon dioxide stack not covered by mask 108. Mask 108is thereafter removed.

In FIG. 5, a phosphorus-doped silicon dioxide 501 is deposited over thesample of FIG. 4. Thereafter, an undoped silicon dioxide 502 isdeposited over phosphorus-doped silicon dioxide 501. In one embodiment,phosphorus-doped silicon dioxide 501 is deposited to a thickness ofabout 500 Angstroms by APCVD, while undoped silicon dioxide 502 isdeposited to a thickness of about 2400 Angstroms also by APCVD. Notethat depending on the application, doped silicon dioxide 501 may also bedoped with N-type dopants other than phosphorus. Phosphorus-dopedsilicon dioxide 501 and undoped silicon dioxide 502 are preferablyconformal to their respective underlying layers.

In FIG. 6, front side 101 is textured using a wet etch processcomprising potassium hydroxide and isopropyl alcohol. The wet etchtextures front side 101 with random pyramids, thereby improving thesolar radiation collection efficiency. Undoped oxide 502 advantageouslyprotects the materials on the back side of wafer 102 from the texturingsolution. In FIG. 6, front side 101 has been relabeled as “101A” toindicate that it has been textured.

In FIG. 7, the sample of FIG. 6 is subjected to in-situ steps. The stepsare in-situ in that they are performed in one loading of wafer 102 intothe wafer processing tool. In one embodiment, the in-situ steps areperformed in a furnace. In the first in-situ step, wafer 102 is heatedto about 1000° C. for one hour to diffuse dopants from boron-dopedsilicon dioxide 104 and phosphorus-doped silicon dioxide 501 into wafer102. This results in the formation of P+ regions 702 (due to boron-dopedsilicon dioxide 104) and N+ regions 703 (due to phosphorus-doped silicondioxide 501) in wafer 102. Note that during the first in-situ step,undoped silicon dioxide 106 beneficially prevents phosphorus fromphosphorus-doped silicon dioxide 501 from diffusing into boron-dopedsilicon dioxide 104. Also, undoped silicon dioxide 502 beneficiallyprevents phosphorus from phosphorus-doped silicon dioxide 501 fromdiffusing into the furnace.

In the second in-situ step, the furnace conditions are changed to about750° C. with an atmosphere containing phosphorus oxychloride, then toabout 975° C. with an atmosphere containing oxygen to diffuse phosphorusas an N-type dopant into wafer 102 from its front side, and to grow athin layer of silicon dioxide on both sides of wafer 102. Front side 101is now labeled as “101B” to indicate that there is a thin layer ofsilicon dioxide on its surface. The thin layer of silicon dioxideadvantageously leads to better passivation of the front side and backside surfaces of wafer 102. In FIG. 7, the front side diffusion ofN-type dopants into wafer 102 is schematically represented by arrows701. As can be appreciated the use of in-situ steps to drive dopantsinto wafer 102 simplifies the fabrication process.

The fabrication of the sample of FIG. 7 may be completed using aback-end process to form electrodes coupled to the P+ and N+ regions ofwafer 102 and to include an anti-reflective coating (e.g., siliconnitride) on front side 101B. For example, a back-end process similar tothat disclosed in U.S. application Ser. No. 10/412,638 may be employedto complete the fabrication of the sample of FIG. 7. Conventional solarcell back-end processes may also be employed without detracting from themerits of the present invention.

While specific embodiments of the present invention have been provided,it is to be understood that these embodiments are for illustrationpurposes and not limiting. Many additional embodiments will be apparentto persons of ordinary skill in the art reading this disclosure.

1. A method of fabricating a back side contact solar cell, the methodcomprising: forming a first layer of material over a back side of asubstrate; forming a second layer of material over the first layer ofmaterial, the second layer of material serving as a diffusion barrier;diffusing a first dopant from the first layer of material to form afirst doped region of a solar cell; diffusing a second dopant into thesubstrate by way of a front side of the substrate; and wherein thediffusion of the first dopant and the second dopant are performedin-situ.
 2. The method of claim 1 further comprising: prior to diffusingthe first dopant from the first layer of material: patterning the firstlayer of material and the second layer of material; forming a thirdlayer of material on the back side of the substrate; forming a fourthlayer of material over the third layer of material, the fourth layer ofmaterial serving as a barrier layer; and diffusing a third dopant fromthe third layer of material to form a second doped region of the solarcell, wherein the third dopant is diffused in-situ with the firstdopant.
 3. The method of claim 2 wherein the third layer of materialcomprises phosphorus-doped silicon dioxide.
 4. The method of claim 1wherein the first layer of material comprises boron-doped silicondioxide.
 5. The method of claim 1 further comprising: forming a printedmask over the first layer of material and the second layer of material.6. The method of claim 5 wherein the mask is formed by screen printing.7. The method of claim 2 wherein the first dopant comprises a P-typedopant, the second dopant comprises an N-type dopant, and the thirddopant comprises an N-type dopant.
 8. A method of fabricating a solarcell, the method comprising: providing a substrate having a front sideand a back side, the front side of the substrate being configured toreceive solar radiation for a solar cell; forming a first doped layerover the back side of the substrate; diffusing a first dopant from thefirst doped layer to form a first doped region of the solar cell; anddiffusing a second dopant from the front side of the substrate in-situwith the diffusion of the first dopant from the first doped layer. 9.The method of claim 8 further comprising: forming a second doped layerover the back side of the substrate; and diffusing a third dopant fromthe second doped layer to form a second doped region of the solar cell,the diffusion of the third dopant from the second doped layer beingperformed in-situ with the diffusion of the first dopant from the firstdoped layer.
 10. The method of claim 8 further comprising: forming afirst undoped layer over the first doped layer to minimize diffusion ofthe third dopant from the second doped layer into the first doped layer.11. The method of claim 10 further comprising: forming a second undopedlayer over the second doped layer to minimize diffusion of the thirddopant from the second doped layer into a processing environment wherethe substrate is heated.
 12. The method of claim 8 wherein the firstdopant from the first doped layer comprises a P-type dopant and thesecond dopant diffused from the front side of the substrate comprises anN-type dopant.